Optical transceiver using heterodyne detection and a transmitted reference clock

ABSTRACT

A heterodyne communication system uses coherent data modulation that is resistant to phase noise. In particular, a pilot tone and reference clock signal are transmitted along with the modulated data to form the basis of an electrical demodulation local oscillator at the receiver. The pilot tone and/or reference clock signal carry phase noise which is correlated with the phase noise in the data signal. At the receiver, the local oscillator is generated from the pilot tone and reference clock signal in a manner so that the local oscillator also has phase noise which is correlated with the phase noise in the data signal. Thus, the two noise components can be used to cancel each other during demodulation of the data signal using the local oscillator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical fiber communications and,more particularly, to a reduction in the effects of phase variationsintroduced by the optical carrier.

2. Description of the Related Art

As the result of continuous advances in technology, particularly in thearea of networking, there is an increasing demand for communicationsbandwidth. For example, the growth of the Internet, home office usage,e-commerce and other broadband services is creating an ever-increasingdemand for communications bandwidth. Upcoming widespread deployment ofnew bandwidth-intensive services, such as xDSL, will only furtherintensify this demand. Moreover, as data-intensive applicationsproliferate and data rates for local area networks increase, businesseswill also demand higher speed connectivity to the wide area network(WAN) in order to support virtual private networks and high-speedInternet access. Enterprises that currently access the WAN through T1circuits will require DS-3, OC-3, or equivalent connections in the nearfuture. As a result, the networking infrastructure will be required toaccommodate greatly increased traffic.

Optical fiber is a transmission medium that is well-suited to meet thisincreasing demand. Optical fiber has an inherent bandwidth which is muchgreater than metal-based conductors, such as twisted pair or coaxialcable. There is a significant installed base of optical fibers andprotocols such as SONET have been developed for the transmission of dataover optical fibers. Typical communications systems based on opticalfibers include a transmitter, an optical fiber, and a receiver.

The transmitter modulates the data, converts the data into an opticalform and transmits the resulting optical signal across the optical fiberto the receiver. In a common design, the transmitter includes a lasersource coupled to an external modulator. The laser source produces anoptical carrier, which is modulated with the data by an externalmodulator. This results in an optical data signal which carries the datato be transported across the optical fiber.

On the other end of the optical fiber, the receiver recovers theoriginal data from the optical data signal transported across theoptical fiber. Recent advances in receiver technology are leading tomore widespread adoption of receivers based on heterodyne detection. Aheterodyne receiver typically includes a local laser source. The locallaser source generates an optical local oscillator which is mixed withthe incoming optical data signal as part of the heterodyne detectionprocess.

To increase the efficiency of data transmission, many fibercommunications systems utilize a coherent modulation scheme beforetransmitting the data on the optical data signal. A coherent modulationscheme takes advantage of phase information in a data signal.

Coherent modulation schemes are used in modulating data onto atransmitted optical signal and demodulating a received signal to obtainthe transmitted data. For coherent modulation, an absolute phasereference is present at the transmitter and receiver to modulate anddemodulated the transmitted signal, respectively. The phase referencesat the transmitter and receiver are said to be phase coherent when theyare locked in phase.

A coherent modulation scheme utilizes phase information in representingdata in a signal. Relative phases errors in the absolute phasereferences generally results in errors in the transmitted data. Examplecoherent modulation schemes include Quadrature Amplitude Modulation(QAM), Phase Shift Keying (PSK) and Quadrature Phase Shift Keying(QPSK).

An electrical signal generated by coherent modulation may be upshiftedto an optical carrier to generate an optical signal employing coherentmodulation. This optical signal can be generated using a laser sourceand an amplitude modulator (e.g. an MZM) to generate coherentlymodulated optical subcarriers. This approach, in which coherentmodulation originates in the electrical domain, contrasts with directcoherent optical modulation, in which optical signals comprisingcoherent modulation can be generated using a phase modulator to modulatethe optical carrier directly.

Data transmitted using coherently modulated optical signals can bereceived using heterodyne detection. Heterodyne detection is a type ofcoherent optical detection, or coherent detection, that generallyutilizes an optical source as a local oscillator to downshift acoherently modulated optical signal to an RF signal. The localoscillator is generally at a different center frequency than an opticalcarrier or subcarrier. Subsequently, the RF signal is demodulated usinga coherent local oscillator at an RF frequency. Data may also becoherently demodulated directly using homodyne detection, in which theoptical local oscillator is phase coherent with the transmitting laser.

One particular optical communication process utilizes coherentmodulation including optical subcarrier multiplexing at the transmitterand heterodyne detection at the receiver. Throughout this process, thedata signal employing coherent modulation is converted from theelectrical domain, to the optical domain, and then back to theelectrical domain. The integrity of the data signal relies on thefidelity of the phase information in the data signal. At each step,phase noise can be introduced into the signal. Significant sources ofphase noise in a transmission system include the phase noise of thetransmit laser source producing an optical carrier and the phase noisein the receive laser source producing the optical local oscillator.Other sources of phase noise include the transmit local oscillatorgenerating the electrical signal and the receive local oscillatordemodulating the received electrical signal.

Traditionally, optical communication systems have addressed the problemof added phase noise by either using a phase-stable laser, or bydesigning complex phase-locking circuits into the demodulator. However,phase-stable lasers are both bulky and expensive, making themimpractical for commercial applications. Complex phase-locking circuitsare likewise expensive and difficult to implement. Furthermore, bothsolutions significantly raise the cost and complexity of the system.

Therefore there is a need for a fiber optic communications system whichadequately deals with phase noise, including laser phase noise, withoututilizing complex phase-locking circuits or expensive lasers.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations of the prior art byproviding a heterodyne communication system using coherent datamodulation that is resistant to phase noise, including laser phasenoise, electrical phase noise, and phase noise from other sources aswell. In particular, a pilot tone and reference clock signal aretransmitted along with the modulated data to form the basis of anelectrical demodulation local oscillator at the receiver end. The pilottone carries phase noise that is correlated with the phase noise in thedata signal. At the receiver, the local oscillator is generated from thepilot tone and reference clock signal in a manner so that the localoscillator also has phase noise that is correlated with the phase noisein the data signal. Thus, the two noise components can be used to canceleach other during demodulation of the data signal using the localoscillator.

In one implementation, a transmitter subsystem includes a pilot tonegenerator, a reference clock generator, a signal generator, and anoptical modulator. The pilot tone generator generates a pilot tone at apilot tone frequency. The reference clock generator generates areference clock signal at a reference clock frequency. The signalgenerator combines the reference clock generator, the pilot tone and asubcarrier channel into an information signal. The subcarrier channelincludes a coherently modulated data stream located at a subcarrierfrequency. In one implementation, the data is modulated according to aQPSK modulation scheme. The optical modulator generates an opticalsignal containing the information signal. Preferably the pilot tone andsubcarrier channel are derived from mutually coherent sources or fromthe same source.

In one embodiment, the signal generator includes a incoherent referencesignal modulator. The incoherent reference signal modulator modulatesthe pilot tone with the reference clock signal utilizing a incoherentmodulation scheme (e.g., AM modulation) to protect the resultingreference signal from phase noise. Thus, the optical signal contains thepilot tone modulated by the reference clock signal and also thesubcarrier channel. In a preferred embodiment, the pilot tone ismodulated using AM modulation and square-law detectors or filtersrecover the reference signal.

In another embodiment, the pilot tone frequency, subcarrier frequency,and reference clock frequency are chosen such that the differencebetween the pilot tone frequency and the subcarrier frequency is afractional multiple of the reference clock frequency. Preferably, thisfractional multiple is an integer multiple. In this way, the pilot tonecan be frequency shifted to the subcarrier frequency by adding (orsubtracting) a multiple of the reference clock.

In another implementation, the signal generator combines additionalsubcarrier channels into the information signal. Each of theseadditional subcarrier channels has a unique subcarrier frequency and iscoherently modulated, preferably by QPSK. Preferably, each subcarrierfrequency can be expressed as a multiple of the reference clockfrequency plus the pilot tone frequency. The use of several uniquesubcarrier frequencies allow the frequency division multiplexing ofseveral data signals while transmitting a single pilot tone andreference clock signal to guard against phase noise.

In another aspect of the invention, a receiver subsystem includes aheterodyne detector, an information signal splitter, a local oscillatorgenerator, and a coherent demodulator. The heterodyne detector receivesan optical signal and converts it into an electrical signal. Theelectrical signal includes a pilot tone, a reference clock signal, and asubcarrier channel. The information signal splitter divides theelectrical signal into these individual components. The local oscillatorgenerator generates an electrical local oscillator from the pilot toneand reference clock signal in a manner such that the local oscillatorcarries phase noise which is correlated with phase noise in thesubcarrier channel. The coherent demodulator demodulates the subcarrierchannel using the local oscillator, canceling the phase noise in the twosignals.

In one implementation, the information signal splitter includes a datarecovery section and a pilot tone recovery section. The data recoverysection isolates the subcarrier channel. The pilot tone recovery sectionisolates the pilot tone. Each recovery section has a corresponding groupdelay, and care is taken to match the two group delays so that, ideally,they are equal, thus providing for cancellation of phase noise.

In another aspect of the invention, the information signal splitterincludes a clock recovery section. The clock recovery section isolatesthe reference clock signal. Preferably, the reference clock signal isincoherently demodulated from the pilot tone in order to remove anyphase noise from the reference clock signal.

In one implementation, the local oscillator generator includes afrequency multiplier and a mixer. The frequency multiplier multipliesthe reference clock frequency by a fractional factor. The mixer adds orsubtracts the multiplied clock from the pilot tone frequency to form thelocal oscillator. Preferably, the reference clock signal does notinclude any phase noise so that when added to the pilot tone, the localoscillator has the same phase noise as the pilot tone, andconsequentially the same phase noise as the subcarrier channel.

In another embodiment, the receiver subsystem includes additional localoscillator generators, additional coherent demodulators and theinformation splitter includes additional data recovery paths. Theinformation splitter recovers additional subcarrier channels located atunique subcarrier frequencies. The additional local oscillatorgenerators generate additional local oscillators, and the additionalcoherent demodulators demodulate the additional subcarrier channels withrespect to the additional local oscillators. Preferably each localoscillator generator generates a local oscillator for each correspondingsubcarrier channel and further contains the same phase noise as thecorresponding subcarrier channel.

In another aspect of the invention the coherent demodulator utilizes aQPSK demodulation scheme. The demodulator includes an I-channel mixer, aQ-channel mixer, two variable phase delays, a Costas phase discriminatorloop and a Quadrature Imbalance Compensated Phase Discriminator loop.The mixers mix the local oscillator and the subcarrier channel togetherto recover the I and Q channels of the data. The variable phase delayswork to adjust the local oscillator to accurately recover the phaseinformation for each channel. The Costas loop adjusts the relative phasebetween the local oscillator and the subcarrier channel while theQuadrature Imbalance Compensated Phase Discriminator adjusts therelative phase between the I and Q channels.

Other aspects of the invention include methods relating to the devicesdescribed above.

One advantage of the present invention is that it significantly reducesthe effects of phase noise in a coherently modulated optical heterodynecommunications system.

BRIEF DESCRIPTION OF THE DRAWING

The invention has other advantages and features which will be morereadily apparent from the following detailed description of theinvention and the appended claims, when taken in conjunction with theaccompanying drawing, in which:

FIG. 1 is a block diagram of a fiber optic communications system usingheterodyne detection.

FIG. 2 is a block diagram of one embodiment of the signal formatter ofFIG. 1.

FIG. 3 is a block diagram of one embodiment of the signal generator ofFIG. 2.

FIG. 4 is a block diagram of one embodiment of the signal extractor ofFIG. 1.

FIG. 5 is a block diagram of one embodiment of the information signalsplitter of FIG. 4.

FIG. 6 is a block diagram of one embodiment of the local oscillatorgenerator of FIG. 4.

FIG. 7 is a flow diagram illustrating one method for selecting varioussignal frequencies and generating an optical data signal.

FIG. 8 is a flow diagram illustrating one method of receiving anddemodulating an optical data signal.

FIG. 9 is a block diagram of one embodiment of the demodulator of FIG.4.

FIG. 10 is a block diagram of one embodiment of a multi-channelcommunications system using heterodyne detection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a fiber optic communications system usingheterodyne detection. System 100 includes a transmitter 110 coupled to areceiver 130 by optical fiber 120. System 100 is used to transmit a datastream 118 from transmitter 110 to receiver 130 via fiber 120.Transmitter 110 includes an optical source 112 coupled to an opticalmodulator 114, and a signal formatter 116 also coupled to opticalmodulator 114. Examples of optical sources 112 include solid state laserand semiconductor lasers. Example optical modulators 114 includeMach-Zender modulators (MZM), electro-optic modulators, andelectro-absorptive modulators. Receiver 130 includes a heterodynedetector 180 coupled to a signal extractor 190. Receiver 130 alsoincludes an optical local oscillator generator 132 for generating anoptical local oscillator signal 134 for use in the heterodyne detector180. Examples of optical LO generator 132 include solid state lasers andsemiconductor lasers.

System 100 operates as follows. Data stream 118 typically is a digitaldata stream. In certain applications, data stream 118 may instead beanalog. Signal formatter 116 receives data stream 118 and formats itinto an information signal 140. An example of one embodiment ofinformation signal 140 can be seen in a frequency spectrum 140′.Spectrum 140′ includes a subcarrier channel 170 characterized by asubcarrier frequency f_(S). The information carried in subcarrierchannel 170 corresponds to the information carried in data stream 118.Spectrum 140′ also includes a pilot tone 172 located at a frequencyf_(t) and two reference clock sidebands 174, 174′ each offset from f_(t)by a reference clock frequency f_(C). The term “tone” as used in thisdescription is contemplated to indicate a continuous wave signal.Spectrum 140′ illustrates one possible arrangement of the elements ofinformation signal 140. Other variations will be apparent to one skilledin the art.

Optical modulator 114 receives information signal 140 and modulates anoptical carrier from optical source 112 to generate an optical datasignal 142. Optical data signal 142 is characterized by an opticalcarrier frequency f_(O). The frequency spectrum of one embodiment ofoptical data signal 142 can be seen in a spectrum 142′. Optical datasignal 142 includes an upper sideband 143U and a lower sideband (notshown or transmitted in the preferred embodiment) containing identicalinformation. Further illustration will reference upper sideband 143U,but the description is equally applicable to the lower sideband. Uppersideband 143U includes a subcarrier channel 144 offset in frequency fromf_(O) by an amount equivalent to f_(S). Upper sideband 143U alsoincludes a pilot tone 146 and two reference clock sidebands 148, 148′.Pilot tone 146 is located at a frequency equal to f_(O)+f_(t), whilereference clock sidebands 148, 148′ maintain an offset of f_(C) frompilot tone 146. Spectrum 142′ illustrates only one possible arrangementof elements in optical data stream 142. Other arrangements will beapparent to one skilled in the art.

In more detail, the optical source 112 produces an optical carriersignal at the optical carrier frequency f_(O). Many commerciallypractical implementations of optical source 112 also introduce asignificant amount of phase noise with the optical carrier signal whichis generally characterized by a linewidth. The modulator 114 receivesthe information signal 140 and modulates the optical carrier with theinformation signal 140 to generate optical data signal 142. FIG. 1illustrates a signal using single sideband modulation resulting in upperside band 143U, however other types of modulation such as doublesideband modulation may also be used. In the example of FIG. 1, themodulator 114 suppresses the optical carrier and does not produce asignificant signal at the optical carrier frequency f_(O). Transmittedpilot tone 146 replaces the optical carrier signal as an aid to signalrecovery within heterodyne detector 180. This arrangement allows thesystem to avoid transmitting the optical carrier signal thus reducingpower handling requirements in fiber 120. Single sideband modulationalso increase spectral efficiency and resistance to impairments such aschromatic dispersion by reducing nonlinear optical distortion.

In a preferred embodiment, the modulator 114 includes a Mach-Zendermodulator (MZM). The conventional two-arm MZM has a raised cosinetransfer function. The MZM is based at one minimum point of the raisedcosine transfer function. Operation at the minimum point suppresses theoptical carrier and the result is an optical data signal that includeslower sideband (not shown) and upper sideband 143U, but no opticalcarrier signal at f_(O). The MZM further includes an optical filter tosuppress the lower sideband.

In another approach, a conventional two-arm MZM is biased at a pointclose to but slightly offset from the minimum point of the raised cosinetransfer function. The slight offset results in some carrier beingintroduced into the optical signal, thus resulting in a spectrum with areduced optical carrier. Other techniques for generating singe sidebandmodulation are discussed in X-cut Lithium Niobate Optical SingleSideband Modulation, K. Higuma et. al., ELECTRONICS LETTERS VOL 37, NO.8, Apr. 12, 2001, which is incorporated by reference in its entiretyherein.

Returning to FIG. 1, the optical data signal 142 and the phase noisefrom optical source 112 are transmitted over fiber 120 to receiver 130.Current optical fibers have two spectral regions which are commonly usedfor communications: the 1.3 and 1.55 micron regions. At a wavelength of1.3 micron, transmission of the optical signal is primarily limited byattenuation in the fiber 120. Dispersion is less of a factor.Conversely, at a wavelength of 1.55 micron, the optical signal willexperience more dispersion but less attenuation. Hence, the opticalsignal preferably has a wavelength either in the 1.3 micron region orthe 1.55 micron region and, for long distance communications systems,the 1.55 micron region is generally preferred.

At receiver 130, heterodyne detector 180 receives the incoming opticaldata signal 142 and also receives an optical local oscillator signal 134at a frequency f_(LO) from optical LO source 132. In FIG. 1, localoscillator signal 134 is shown at a frequency f_(LO) which is equal tothe carrier frequency f_(O). However, the local oscillator signal 134may also be located at a frequency f_(LO) which is higher or lower thanthe carrier frequency f_(O). The optical signal 142 and local oscillatorsignal 134 are combined and heterodyne detection of the combined signaleffectively downshifts optical signal 142 from a carrier at frequencyf_(O) to the original frequency space of information signal 140. Theresulting electrical signal 150 has spectrum 150′. Note that uppersideband 153U has been recovered and frequency downshifted compared tooptical signal 142. In the preferred embodiment, sideband 153U includesa subcarrier channel 152, a pilot tone 154 and two reference clocksidebands 156, 156′. These elements correspond to the elements presentin information signal 140. Electrical signal 150 also contains the phasenoise generated by optical source 112.

Signal extractor 190 receives electrical signal 150 from heterodynedetector 180, and removes the formatting applied to data stream 118 bysignal formatter 116. Signal extractor 190 then outputs a recovered datastream 195 which carries the same information and format as data stream118.

FIG. 2 shows one embodiment 216 of signal formatter 116. Signalformatter 216 includes a data modulator 210 which receives data stream118, a subcarrier frequency generator 220 coupled to data modulator 210,a signal generator 240 coupled to data modulator 210, and a pilot tonegenerator 250 and a reference clock generator 260 both coupled to signalgenerator 240.

Signal formatter 216 operates as follows. Data stream 118 is received bydata modulator 210. Data modulator 210 includes a coherent modulator tomodulate data stream 118 into subcarrier channel 230 which ischaracterized by a subcarrier frequency f_(S) provided by subcarrierfrequency generator 220. In one embodiment, data modulator 210 utilizesa QAM or QPSK modulation scheme, but one skilled in the art willrecognize that other coherent modulation schemes are equally applicable.Coherent modulation provides an advantage over other modulationtechniques by better using available bandwidth through use of a signal'sphase information. However, a coherently modulated signal is susceptibleto the phase noise generated by optical source 112 as well as any phaseerror introduced into system 100. In order to keep system 100 efficient,the effects of phase noise and phase error should be reduced. As willbecome evident in the discussion below, the present invention includesnovel and advantageous ways of efficiently reducing the effects of phasenoise and/or phase error.

Returning to FIG. 2, signal generator 240 receives subcarrier channel230 from data modulator 210 as well as a pilot tone 255 at a frequencyf_(t) and a reference clock signal 265 at a frequency f_(C) from pilottone generator 250 and reference clock generator 260, respectively.Pilot tone generator 250, reference clock generator 260 and subcarrierfrequency generator 220 are all conventionally implemented frequencygenerators and may consist phase-locked oscillators and multipliers,conventional crystal oscillators, astable vibrator circuits, digitalsignals, or other suitable frequency generators. Signal generator 240combines the signals from data modulator 210, pilot tone generator 250and reference clock generator 260 to produce information signal 140.

As noted above, spectrum 140′ illustrates just one arrangement of thesignals into information signal 140. By adjusting the frequenciesgenerated by subcarrier frequency generator 220, pilot tone generator250, and reference clock generator 260 different arrangements of thesignals within information signal 140 can be generated. Furthermore, thepilot tone 255, reference clock signal 265 and subcarrier channel 230can be combined in different ways to generate different informationsignals 140.

FIG. 3 illustrates one embodiment 340 of signal generator 240. Signalgenerator 340 includes a reference signal modulator 310 coupled to asignal combiner 320. Reference signal modulator 310 receives referenceclock signal 265 and pilot tone 255 and modulates pilot tone 255 withreference clock signal 265 to generate reference signal 350. Thefrequency spectrum of a preferred embodiment of reference signal 350 isillustrated in a spectrum 350′. Spectrum 350′ includes a pilot tone 352at frequency f_(t), and two reference clock sidebands 354, 354′ offsetfrom f_(t) by f_(C) resulting from the modulation. Signal combiner 320receives reference signal 350 and subcarrier channel 230 and combinesthe signals to generate information signal 140. Signal combiner 320 mayinclude a signal adder or other commonly known method for combining thesignals into information signal 140.

In more detail, reference signal modulator 310 includes a non-coherentmodulator. Examples of non-coherent modulation include AM modulation. Asnoted above, phase noise can interfere with the coherent modulationscheme utilized in data modulator 210. Therefore it is desirable to finda way to cancel or de-emphasize the effects of the phase noise on signalextractor 190. As will be shown below, pilot tone 255 and referenceclock signal 265 are used by signal extractor 190 to help defeat thephase noise present at signal extractor 190. To aid in this process,modulator 310 utilizes an amplitude modulation scheme to modulate pilottone 255 with reference clock signal 265. One skilled in the art willrecognize that other non-coherent modulation schemes may also be used.By using an amplitude modulation scheme, reference clock signal 265 canbe recovered by signal extractor 190 without interference from the phasenoise.

FIG. 4 is a block diagram of an embodiment 490 of signal extractor 190.Signal extractor 490 includes an information signal splitter 405,coupled to a local oscillator generator 450 and to a demodulator 460.Local oscillator generator 450 is also coupled to demodulator 460. Inone embodiment, information signal splitter 405 includes an electricalsignal splitter 410, a tone recovery arm 420, a reference clock recoveryarm 430, and a data recovery arm 440. Electrical signal splitter 410 isconfigured to receive electrical signal 150 and is coupled to eachrecovery arm 420, 430, and 440. Tone recovery arm 420 and clock recoveryarm 430 are both coupled to local oscillator generator 450. Datarecovery arm 440 and local oscillator generator 450 each output todemodulator 460 which outputs recovered data stream 195.

Signal extractor 490 operates as follows. Electrical signal 150 isreceived from heterodyne detector 180 by electrical signal splitter 410.Electrical signal splitter 410 generates three copies 150A, 150B, and150C, of electrical signal 150, one for each recovery arm 420, 430, and440. Data recovery arm 440 isolates the subcarrier channel 445,characterized by subcarrier frequency f_(S) from electrical signal copy150C and outputs subcarrier channel 445 along with the phase noise todemodulator 460. Tone recovery arm 420 isolates the pilot tone frequencycomponent 154 from its electrical signal copy 150A and outputs a pilottone component 425 at frequency f_(t) and the phase noise to localoscillator generator 450. Reference clock recovery arm 430 isolates thepilot tone 154 and reference clock components 156, 156′ from electricalsignal copy 150B and outputs a reference clock signal 435 at frequencyf_(C) without phase noise to local oscillator generator 450. Localoscillator generator 450 receives both pilot tone 425 and referenceclock 435 and generates an electrical local oscillator 455 at frequencyf_(OSC).

In the preferred embodiment f_(OSC) is equal to f_(S). This feature isparticularly useful if f_(O) is equal to f_(LO) in heterodyne detector180 so that electrical signal occupies the same frequency spaces asinformation signal 140. If f_(O) is not equal to f_(LO), it would benecessary to adjust f_(OSC) to account for the incomplete downshiftingof optical data signal 142 at heterodyne detector 180. Such adjustmentsare known to those skilled in the art and are contemplated within thescope of this invention. Local oscillator 455 has the same phase noiseand group delay as subcarrier channel 445.

Specifically, in signals 150A-C, the pilot tone 154 and subcarrierchannel 152 carry correlated phase noise since they were generated andtransmitted together in transmitter 110 and heterodyne detected togetherin receiver 130. The phase noise in the pilot tone 154 traverses asignal path through the tone recovery arm 420 and local oscillatorgenerator 450 to reach demodulator 460. The phase noise in thesubcarrier channel 152 traverses a signal path through the data recoveryarm 440 to reach demodulator 460. These two signal paths are matched sothat the phase noise experiences the same group delay. Thus, when theyarrive at demodulator 460, they will still be correlated and can becanceled at demodulator 460.

Returning to the general operation of signal extractor 490, localoscillator 455 is output to demodulator 460. Demodulator 460 demodulatessubcarrier channel 445 with respect to local oscillator 455 and outputsrecovered data stream 195. In a preferred embodiment (f_(O)=f_(LO) atheterodyne detection) when local oscillator 455 has the same frequencyf_(OSC) as subcarrier channel 445, namely f_(S), the recovered data 195is completely downshifted to the original form of data stream 118. Inembodiments utilizing a heterodyne detection where f_(O) does not equalf_(LO), an adjusted local oscillator frequency f_(OSC) will be necessaryto completely downconvert recovered data stream 195.

Demodulator 460 utilizes the same modulation scheme as used in signalformatter 116. In a preferred embodiment, demodulator 460 uses the samecoherent demodulation scheme as data modulator 210. As noted in thediscussion of data modulator 210, coherent demodulation schemes aresusceptible to phase noise introduced by optical source 112 as well asany group delays within the signal recovery arms 420, 430, and 440.However, if the local oscillator 455 and data signal 445 include thesame phase noise and group delay, then the phase distortions will cancelwithin demodulator 460 allowing efficient recovery of recovered datasignal 195. As noted above, local oscillator generator 450 and datarecovery arm 440 are designed to produce signals with correlated phaseerror and phase delay.

FIG. 5 is a block diagram of one embodiment of information signalsplitter 405 including specific embodiments 502, 504 and 506 of clockrecovery arm 430, tone recovery arm 420 and data recovery arm 440,respectively. In detail, clock recovery arm 502 includes a band passfilter 510 coupled to a square-law device 520, coupled to a low passfilter 530. Band pass filter 510 receives electrical signal copy 150Band isolates pilot tone 154 and at least one reference clock sideband156, 156′. Square-law device 520 and low pass filter 530 are oneimplementation of an AM demodulator. They recover a reference clocksignal 435 at frequency f_(C) equal to the difference between pilot tone154 and reference clock sideband 156. More specifically, square-lawdevice 520 multiplies the incoming signal against itself. Low passfilter 530 filters out any higher frequency additive terms and recoversthe low frequency difference term. As noted above and in the discussionof FIG. 3, by taking the difference term at low pass filter 530, thesignal is effectively stripped of any phase noise and phase error fromthe reference clock signal. In other embodiments clock recovery arm 430may include other structures known in the art to implement anon-coherent demodulation scheme corresponding to the scheme used byreference signal modulator 310.

Tone recovery arm 504 includes a band pass filter 540 configured toisolate pilot tone 154 from electrical signal 150A. Likewise, datarecovery arm 506 includes a band pass filter 550 configured to isolatesubcarrier channel 152 from electrical signal 150C. Both recovery arms504, 506 preserve the phase noise introduced into optical data signal142 by optical source 112. While recovery arms 540 and 550 areillustrated as using band pass filters, other methods for isolating therespective frequency components can be used.

FIG. 6 is a block diagram of an embodiment 650 of local oscillatorgenerator 450. Local oscillator generator 650 includes a frequencymultiplier 610 coupled to a local oscillator combiner 620. In oneembodiment, local oscillator combiner 620 includes signal multiplier 630coupled to band pass filter 640.

Local oscillator generator 650 operates as follows. Frequency multiplier610 receives reference clock signal 435 at frequency f_(C). Frequencymultiplier 610 generates a multiplied clock 615 which is at a frequencywhich is a fractional multiple N of the frequency of the incomingreference clock 435. In other words the frequency of reference clocksignal 435 is increased from f_(C) to N·f_(C). In a preferred embodimentN is an integer factor. However, any reproducible factor can be chosen.Local oscillator combiner 620 combines pilot tone 425 and multipliedclock 615 to generate local oscillator 455 having the same phase noiseand group delay as subcarrier channel 445. In a preferred embodiment,group delay matching is accomplished by ensuring that the signal pathsare matched from separation to reconstruction.

In the embodiment shown in FIG. 6, signal multiplier 630 receives pilottone 425 and multiplied clock 615. These two signals are multipliedtogether to form a difference signal and a sum signal. The differencesignal is located at a difference between the frequencies of the tone425 and multiplied clock 615. The sum signal is located at a sum of thefrequencies of the tone 425 and multiplied clock 615. Note that thephase noise present with pilot tone 425 is preserved in both the sumsignal and the difference signal since the reference clock signal 435(and therefore also the multiplied clock signal 615) does not have aphase noise component. Generally, the phase noise comprises both opticaland electrical phase noise. In DWDM systems, the phase noise may furthercomprise cross-phase modulation from other wavelengths carrying datasignals as well as other phase noise arising from nonlinear opticalcoupling of two or more wavelength channels. The sum and differencesignals are passed to band pass filter 640, which isolates one of thetwo signals for output as local oscillator 455. In effect, tone 425 iseither shifted up or down in frequency by multiplied clock 615 to formlocal oscillator 455 depending on whether f_(t) is higher or lower thanf_(S). The process for selecting N as well as for selecting f_(t),f_(C), and f_(S) will be discussed below.

FIG. 7 is a flow diagram of a method 700 for selecting the varioussignal frequencies f_(C), f_(t), and f_(S), and generating optical datasignal 142. Reference numerals corresponding to FIGS. 1-6 have beenincluded in parenthesis to provide an example of a specific embodimentof method 700. In method 700, a data stream (118) is received 710 and asubcarrier (220) is generated 720. The data stream is coherentlymodulated 730 onto the subcarrier (210, 230). In one embodiment, thesubcarrier may be also be upshifted 735 according to a WavelengthDivision Multiplexing scheme. In addition, both a pilot tone (250, 255)and a reference clock (260,265) are generated 740 and 750, respectively.The pilot tone is non-coherently modulated 760 with the reference clocksignal (310, 350) to resist the effects of phase noise at a receiver(130). The modulated pilot tone and modulated data stream are combined770 into an information signal (320, 140), which is opticallytransmitted 780.

In more detail, generation of the subcarrier, pilot tone and referenceclock and the selection of their corresponding are interrelated with theoverall design goals of the system. A change in the pilot tone frequencyf_(t) typically will change the reference clock frequency f_(C) and/orthe subcarrier frequency f_(S).

As noted above in the discussion of local oscillator combiner 620, inone embodiment, a general formula for the interrelationship between thefrequencies is as follows. f_(S)=f_(t)+/−N·f_(C), where N is somefraction and preferably an integer. In embodiments where the opticallocal oscillator (132) of the heterodyne detector (180) does not havethe same frequency f_(LO) as the optical carrier f_(O) generated by theoptical source 112, the combination of N·f_(C)+/−f_(t) should beadjusted accordingly to account for the offset between f_(LO) and f_(O).The pilot tone (265) is selected to be at a frequency f_(t) suitable toform a base from which to generate local oscillator (455) at thedemodulator (460). Furthermore, the pilot tone located at frequencyf_(t) should reside outside of a bandwidth defined by the subcarrierchannel. The reference clock frequency and multiplied clock frequencyare chosen to raise or lower the pilot tone (255) so that the resultinglocal oscillator frequency f_(OSC) is equal to the subcarrier frequencyf_(S). Typically f_(t) is close in frequency to f_(S) while f_(C) is amuch lower frequency.

This arrangement provides several advantages By having a smaller f_(C),when the pilot tone (255) is modulated with the reference clock (265),the resulting reference signal (350) has a smaller bandwidth. Thispromotes efficiency in transmitting the optical data signal (142) byallowing the pilot tone (146) and reference clock sidebands (148, 148′)to fit in the frequency spectrum closer to the subcarrier channel (144).Furthermore, a small f_(C) increases the scalability since severalsubcarrier frequencies can be used and several local oscillators can bematched to these subcarrier frequencies by simply changing the value ofN for each subcarrier frequency. This particular embodiment will bediscussed in more detail in conjunction with FIG. 10 below.

FIG. 8 is a flow diagram illustrating one method 800 of receiving anddemodulating optical data signal 142. Reference numerals correspondingto FIGS. 1-6 have been included in parenthesis to provide an example ofa specific embodiment of method 800. An optical signal (142) is received810 and converted 820 to an electrical signal (150) while preserving thephase noise on the signal (142). The pilot tone (154) in the electricalsignal is incoherently demodulated 830 to generate a reference clocksignal (435) without phase noise. The pilot tone (425) is also recovered840, but with phase noise. A coherently modulated data stream (445) isalso recovered 850 with a phase noise correlated to the phase noise inthe pilot tone. The recovered clock signal is frequency multiplied 860to generate a multiplied clock signal (615), which is mixed 870 with therecovered pilot tone (425) to produce an electrical local oscillator(455) with the same phase noise as in the pilot tone. The coherentlymodulated data stream (445) is coherently demodulated 880 with respectto the recovered local oscillator (425) to produce a recovered datastream (195). In the demodulation process, the phase noise in the datastream and the local oscillator cancel. The recovered data stream (195)is output 890.

FIG. 9 illustrates a preferred embodiment 900 of demodulator 460.Demodulator 900 includes a Q-mixer 905, an I-mixer 910, a low passfilter 915, a second low pass filter 920, a variable phase delay 930, asecond variable phase delay 950, a quad imbalance phase discriminator960, a quad imbalance offset controller 970, a Costas loop phasediscriminator 980 and a Costas loop offset controller 990. Subcarrierchannel 445 is received by both Q-mixer 905 and I-mixer 910, whichoutputs are filtered through low pass filters 920 and 915, respectively.The output from the low pass filters 920 and 915 constitute the outputfrom demodulator 900, which are the recovered Q and I channels,respectively.

Local oscillator 455 is received by variable phase delay 930, whichselectively increases or decreases the phase of local oscillator 455.Variable phase delay 930 outputs the phase delayed local oscillator toI-mixer 910 and to second variable phase delay 950. Second variablephase delay 950 adds approximately 90° of phase to the phase delayedlocal oscillator and outputs the signal to Q-mixer 905. Quad imbalancephase discriminator 960 receives the output from both low pass filters920, 915. Quad imbalance phase discriminator 960 is coupled to quadimbalance offset controller 970 which in turn is coupled to variablephase delay 950. Similarly, Costas loop phase discriminator 980 receivesthe output from both low pass filters 920, 915 and is coupled to Costasoffset controller 990 which in turn is coupled to variable phase delay930.

Demodulator 900 operates as follows. Demodulator 900 coherentlydemodulates subcarrier channel 445 to produce an I channel and a Qchannel of recovered data 195. In a preferred embodiment the modulationis QPSK. However, other forms of modulation may also be used, such asQuadrature Amplitude Modulation (QAM), Frequency Shift Keying (FSK),Amplitude Shift Keying (ASK), Phase Shift Keying (PSK), BiPhase ShiftKeying (BPSK), and On Off Keying (OOK). Generally, in QPSK, two datachannels are modulated by subcarrier signals which have the samereference frequency, but are 90° out of phase, allowing the signals tobe combined without significant interference for more efficient use ofbandwidth. At the demodulation stage, such as demodulator 900, thecombined signal is again mixed with the two 900 out-of-phase referencesignals reproducing the original two data channels.

In a preferred embodiment, subcarrier channel 445 is input into Q-mixer905 and I-mixer 910. In mixers 905 and 910, subcarrier channel 445 ismixed with two variants of local oscillator 455, 455A and 455B, torecover the Q and I channels respectively. As with most demodulationschemes, the resultant signals from Q and I mixers 905, 910 contains adifference component and a sum component. Low pass filters 920, 915filter the outputs of Q-mixer 905 and I-mixer 910 respectively to allowonly the difference components to be output. By selecting the differencecomponent, phase noise is effectively cancelled.

Local oscillator 455 is received into the demodulator by variable phasedelay 930. Variable phase delay 930 selectively adjusts the phase onlocal oscillator 455 to generate 455B and passes the signal 455B toI-mixer 910 via second variable phase delay 950. While depicted in FIG.9 as an external local oscillator signal 455 coupled to the variablephase delay 930, local oscillator 455B may more generally be generateddirectly by an oscillator 940. One such oscillator 940 would be avoltage controlled oscillator (VCO) which receives the output of theCostas Offset Controller 990 as control input. Second variable phasedelay 950 is a typical component in any basic QPSK demodulator. Secondvariable phase delay 950 typically adds approximately an additional 90°of phase to local oscillator 455B but may selectively adjust the amountof phase added to local oscillator 455B to produce 455A which is used byQ-mixer 905 to demodulate the Q channel from subcarrier channel 445.

Variable phase delays 930 and 950 operate as follows. Variable phasedelay 930 adjusts the phase of local oscillator 455 to match the phaseof subcarrier channel 445. To control this adjustment, demodulator 900uses Costas loop phase discriminator 980. Costas loop phasediscriminator 980 generates an error signal 985 from sampling the I andQ channels from the output of low pass filters 915 and 920. Error signal985 is calculated based on the quantity <I·|Q|>−<Q·|I|> where I and Qare outputs of the quadrature demodulator and the operator < > indicatesa time average and | | indicates a Signum function. The error signal 985is a measure of the phase offset between the local oscillator phase andthe phase of the incoming signal. Costas loop offset controller 990 useserror signal 985 to control variable phase delay 930. The goal of Costasloop offset controller 990 is to adjust the phase so that error signal985 is driven to zero.

Variable phase delay 950 adjusts the phase of local oscillator 455B tomatch the amount of phase difference between the I and Q channels. Asnoted above, in ideal conditions the I and Q channels are modulated 90°out of phase. Ideally, 90° of phase should be added by second variablephase delay 950 to efficiently recover the I and Q channels. However,operating conditions may exist where the I and Q channels may be more orless than 90° out of phase. Variable phase delay 950 effectively finetunes the phase shifting to match operating conditions. To control thisadjustment demodulator 900 uses quad imbalance phase discriminator 960.Quad imbalance phase discriminator 960 generates an error signal 965from sampling the I and Q channels from the output of low pass filters915, and 920. Error signal 965 is calculated based on the quantity<I·|Q|>+<Q·|I|>. This error signal 965 measures quadrature imbalance,i.e. the extent to which phase offset between I and Q differs from π/2.Quad imbalance offset controller 970 uses error signal 965 to controlvariable phase delay 950. The goal of quad imbalance offset controller970 is to adjust the relative phase enough so that error signal 965 isdriven to zero.

FIG. 10 illustrates one embodiment 1000 of a multi-channelcommunications system using heterodyne detection. In system 100 of FIG.1, there was a single subcarrier channel 170 and a single correspondingpilot tone 172 and reference clock signal 174. In system 1000, there aremany subcarrier channels 1170 which share a common pilot tone 1172 andreference clock signal 1174.

On the transmit side, communication system 1000 includes a plurality ofdata modulators 1210A-D, a plurality of corresponding subcarrierfrequency generators 1220A-D, a signal combiner 1320, an opticalmodulator 1114 an optical source 1112 and an optical fiber 1120. On thereceive side, communication system 1000 includes a heterodyne detector1180, an optical local oscillator generator 1132, a signal powersplitter 1410, a tone recovery arm 1420, a clock recovery arm 1430, aplurality of data recovery arms 1440A-D, a plurality of correspondinglocal oscillator generators 1450A-D, and corresponding demodulators1460A-D.

Modulators 1210A-D each receive a unique subcarrier frequency fromsubcarrier frequency generators 1220A-D. Modulators 1210A-D are coupledto signal combiner 1320 which in turn is coupled to optical modulator1114. Optical modulator 1114 also receives an optical carrier fromoptical source 1112 and is coupled to optical fiber 1120. Heterodynedetector 1180 receives input from fiber 1120 and optical localoscillator 1132 and outputs to signal power splitter 1410. Signal powersplitter 1410 outputs to tone recovery arm 1420, clock recovery arm 1430and data recovery arms 1440A-D. Tone recovery arm 1420 and clockrecovery arm 1430 are coupled to each of the local oscillator generators1450A-D. Data recovery arms 1440A-D and local oscillator generators1450A-D are pairwise matched and coupled to their respectivedemodulators 1460A-D.

Communication system 1000 operates as follows. Each individual partoperates as discussed above. For ease of recognition, similar numbershave been used where appropriate. Furthermore, elements unique to asingle data channel have been designated with an A-D suffix. Elementshaving the same suffix work together to provide a single data channel.Generally, modulators 1210A-D each operate to modulate a separate datastream (not shown) onto a unique subcarrier frequency f_(Cx) provided bysubcarrier frequency generators 1220A-D respectively. The outputs frommodulators 1210A-D represent a plurality of data channels 1230A-D.

Reference signal modulator 1310 operates as discussed with respect toFIG. 3. Reference signal modulator 1310 modulates a pilot tone frompilot tone generator 1250 with a reference clock from reference clockgenerator 1260 to form reference signal 1350. Data channels 1230 A-D andreference signal 1350 are combined by signal combiner 1320 to form asingle frequency multiplexed information signal 1140. Information signal1140 has a plurality of subcarrier channels 1170A-D each correspondingto a data channel 1230A-D. Information 1140 signal further includes apilot tone 1172 and a pair of reference clock sidebands 1174, 1174′,corresponding to reference signal 1350. Information signal 1140 isreceived by optical modulator 1114 which optical transmits informationsignal 1140 as an optical signal 1142 on an optical carrier provided byoptical source 1112 over fiber 1120 and is received by heterodynedetector 1180.

Using optical local oscillator 1132, heterodyne detector 1180 receivesand converts optical signal 1142 into an electrical signal 1150.Electrical signal 1150 is split into a plurality of identical signals bysignal power splitter 1410. Tone recovery arm 1420, clock recovery arm1430, and data recovery arms 1440A-D each receive a split signal. Tonerecovery arm 1420 recovers the pilot tone. Clock recovery arm 1430recovers the reference clock. Each data recovery arm 1440A-D isolatesits designated subcarrier channel A-D. Tone recovery arm 1430 and clockrecovery arm 1430 provide input to each local oscillator generators1450A-D.

As in the single channel embodiment, local oscillator generators 1450A-Dmultiply the reference clock signal by N and mix it with the pilot tonesignal to generate a f_(OSC) equal to f_(S). In communication system1000, however, each local oscillator 1450A-D has a unique N tofacilitate using the same clock signal and pilot tone signal to generateunique local oscillators matching their respective subcarrierfrequencies generated by subcarrier frequency generators 1220A-D. Interms of the formulas noted above in the discussion of FIGS. 7 and 8,the N_(x) is chosen such that: f_(t)+/−N_(x)·f_(C)=f_(OSCx)=f_(Sx) wherex designates one of the data channels A-D. Demodulators 1460A-D may beidentical in operation and construction. Demodulators 1460A-D receivethe unique local oscillators from local oscillator generators 1450A-Dand their corresponding subcarrier channels from data recovery arms1440A-D, and demodulate the subcarrier channels with respect to localoscillators 1450A-D to recover the original data channels.

Although the invention has been described in considerable detail withreference to certain embodiments thereof, other embodiments arepossible. Therefore, the scope of appended claims should not be limitedto the description of the preferred embodiments contained herein.

1. An optical communications system comprising: a transmitter subsystemcomprising: a pilot tone generator for generating a pilot tone at apilot tone frequency; a reference clock generator for generating areference clock signal at a reference clock frequency; a signalgenerator coupled to the pilot tone generator and the reference clockgenerator, wherein the signal generator includes an incoherent referencesignal modulator configured to generate a reference signal by using thereference clock signal to modulate the pilot tone, and wherein thesignal generator is configured to combine the reference signal with asubcarrier channel into an information signal; and an optical modulatorcoupled to the signal generator for generating an optical signal, theoptical signal including the reference signal and the subcarrierchannel; wherein the signal generator is further configured to combineadditional subcarrier channels into the information signal, wherein eachsubcarrier channel includes a coherently modulated data stream and islocated at a unique subcarrier frequency.
 2. The optical communicationssystem of claim 1 wherein the subcarrier channel includes a coherentlymodulated data stream and is located at a subcarrier frequency.
 3. Theoptical communications system of claim 1 wherein in generating-theoptical signal, the optical modulator preserves a phase relationshipbetween the pilot tone and the subcarrier channel.
 4. The opticalcommunications system of claim 1 wherein a power in the reference clocksignal of the optical signal is substantially lower than 30 dB down froma total power in the optical signal.
 5. The optical communicationssystem of claim 1 wherein within the information signal the frequencyspace occupied by the reference signal does not overlap with thefrequency space occupied by the subcarrier channel.
 6. The opticalcommunications system of claim 1 wherein the signal generator furthercomprises: a signal combiner coupled to the incoherent reference signalmodulator for combining the reference signal and the subcarrier channel.7. The optical communications system of claim 1 wherein the incoherentreference signal modulator includes an AM modulator.
 8. The opticalcommunications system of claim 1 wherein the reference clock signal is atone.
 9. The optical communications system of claim 1 wherein thedifference between the pilot tone frequency and the subcarrier frequencyis a fractional multiple of the reference clock frequency.
 10. Theoptical communications system of claim 9 wherein the fractional multipleis an integer multiple.
 11. The optical communications system of claim 1wherein the subcarrier channel includes a QPSK modulated data stream.12. The optical communications system of claim 1 wherein the signalgenerator comprises: a signal combiner coupled to the incoherentreference signal modulator for combining the reference signal and thesubcarrier channels; wherein within the information signal, thefrequency space occupied by the reference signal does not overlap withthe frequency space occupied by the subcarrier channel.
 13. The opticalcommunications system of claim 12 wherein the incoherent referencesignal modulator includes an AM modulator.
 14. The opticalcommunications system of claim 1 wherein the difference between thepilot tone frequency and each subcarrier frequency is a fractionalmultiple of the reference clock frequency.
 15. The opticalcommunications system of claim 14 wherein the fractional multiple is aninteger multiple.
 16. The optical communications system of claim 1wherein the inclusion of the pilot tone and the reference clock signalin the optical signal does not appreciably increase a bandwidth occupiedby the optical signal.
 17. An optical communications system comprising:a transmitter subsystem comprising: a pilot tone generator forgenerating a pilot tone at a pilot tone frequency; a reference clockgenerator for generating a reference clock signal at a reference clockfrequency; a signal generator coupled to the pilot tone generator andthe reference clock generator for combining the pilot tone, thereference clock signal and a subcarrier channel into an informationsignal; and an optical modulator coupled to the signal generator forgenerating an optical signal, the optical signal including the pilottone, the reference clock signal, and the subcarrier channel; and areceiver subsystem comprising: a heterodyne detector for receiving anoptical signal and converting it to an electrical information signal,the electrical information signal comprising a pilot tone, a referenceclock signal, and a subcarrier channel; an information signal splittercoupled to the heterodyne detector for dividing the information signalinto the pilot tone, the reference clock signal, and the subcarrierchannel; a local oscillator generator coupled to the information signalsplitter for generating a local oscillator from the pilot tone and thereference clock signal; a demodulator coupled to the information signalsplitter and the local oscillator generator, for demodulating thesubcarrier channel using the local oscillator; and wherein, at thedemodulator, phase noise contained in the local oscillator is correlatedwith phase noise contained in the subcarrier channel.
 18. The opticalcommunications system of claim 17 wherein the demodulator comprises acoherent demodulator.
 19. The optical communications system of claim 17wherein the phase noise comprises one or more from the list of: laserphase noise; electrical phase noise; and phase noise from nonlinearoptical coupling between two or more wavelength channels.
 20. Theoptical communications system of claim 17 wherein the phase noise is theresult of cross phase modulation.
 21. An optical communications systemcomprising: a receiver subsystem comprising: a heterodyne detector forreceiving an optical signal and converting it to an electricalinformation signal, the electrical information signal comprising a pilottone at a pilot tone frequency, a reference clock signal at a referenceclock frequency, and a subcarrier channel located at a subcarrierfrequency; an information signal splitter coupled to the heterodynedetector for dividing the information signal into the pilot tone, thereference clock signal, and the subcarrier channel; a local oscillatorgenerator coupled to the information signal splitter for generating alocal oscillator from the pilot tone and the reference clock signal; ademodulator coupled to the information signal splitter and the localoscillator generator, for demodulating the subcarrier channel using thelocal oscillator; and wherein, at the demodulator, phase noise containedin the local oscillator is correlated with phase noise contained in thesubcarrier channel.
 22. The optical communications system of claim 21,wherein the demodulator comprises a coherent demodulator.
 23. Theoptical communications system of claim 21, wherein the informationsignal splitter is configured to upshift and downshift the pilot tone.24. The optical communications system of claim 21, wherein the phasenoise comprises one or more from the list of: laser phase noise;electrical phase noise; and phase noise from nonlinear optical couplingbetween two or more wavelength channels.
 25. The optical communicationssystem of claim 21, wherein the phase noise is the result of cross phasemodulation.
 26. The optical communications system of claim 21 wherein:the information signal splitter comprises: a data recovery section forrecovering the subcarrier channel from the information signal; and pilottone recovery section for recovering the pilot tone from the informationsignal; and wherein a group delay through the data recovery section tothe coherent demodulator is matched to a group delay through the pilottone recovery section and local oscillator generator to the coherentdemodulator.
 27. The optical communications system of claim 26 whereinthe group delays are matched by matching a path length through the pilottone recovery section and local oscillator generator to a path lengththrough the data recovery section.
 28. The optical communications systemof claim 21 wherein the information signal sputter comprises: areference clock recovery section for recovering the reference clocksignal from the information signal, wherein the recovered referenceclock signal is substantially free from phase noise.
 29. The opticalcommunications system of claim 28, wherein the phase noise may beintroduced at the receiver or at a transmitter.
 30. The opticalcommunications system of claim 28 wherein: within the electricalinformation signal, the reference clock signal is incoherently modulatedonto the pilot tone; and the reference clock recovery section comprises:a bandpass filter tuned to extract the modulated pilot tone from theelectrical information signal; a square-law device coupled to thebandpass filter to demodulate the reference clock signal from the pilottone, and a lowpass filter coupled to the square-law device forrecovering the reference clock signal.
 31. The optical communicationssystem of claim 30 wherein, within the electrical information signal,the reference clock signal is AM modulated onto the pilot tone.
 32. Theoptical communications system of claim 21 wherein the local oscillatorgenerator comprises: a frequency multiplier coupled to the informationsignal splitter for multiplying the reference clock signal by afraction; and a mixer coupled to the frequency multiplier and theinformation signal splitter, for mixing the multiplied reference clocksignal with the pilot tone to produce the local oscillator.
 33. Theoptical communications system of claim 32 wherein the fraction is aninteger.
 34. The optical communications system of claim 32 wherein: thereference clock signal is substantially free from phase noise; and thelocal oscillator signal contains phase noise substantially identical tophase noise contained in the pilot tone.
 35. The optical communicationssystem of claim 32 wherein a frequency of the local oscillator is equalto the subcarrier frequency.
 36. The optical communications system ofclaim 21 wherein: at the coherent demodulator, the phase noise containedin the local oscillator substantially cancels the phase noise containedin the subcarrier channel.
 37. The optical communications system ofclaim 21 wherein: the electrical information signal further comprisesadditional subcarrier channels, wherein each subcarrier channel islocated at a unique subcarrier frequency; the information signalsplitter further divides the information signal into the subcarrierchannels; the receiver subsystem further comprises: additional localoscillator generators, each local oscillator generator coupled to theinformation signal splitter for generating an additional localoscillator; additional demodulators, each demodulator coupled to theinformation signal splitter and the local oscillator generator, fordemodulating one of the additional subcarrier channels using one of theadditional local oscillators; and wherein, at each demodulator, phasenoise contained in the additional local oscillator is correlated withphase noise contained in the additional subcarrier channel.
 38. Theoptical communications system of claim 37 wherein each local oscillatorgenerator comprises: a frequency multiplier coupled to the informationsignal splitter for multiplying the reference clock signal by afraction; and a mixer coupled to the frequency multiplier and theinformation signal splitter, for mixing the multiplied reference clocksignal with the pilot tone to produce the local oscillator.
 39. Theoptical communications system of claim 38 wherein: the reference clocksignal is substantially free from phase noise; and the local oscillatorsignal contains phase noise substantially identical to phase noisecontained in the pilot tone.
 40. The optical communications system ofclaim 38 wherein a frequency of each local oscillator is equal to thecorresponding subcarrier frequency.
 41. The optical communicationssystem of claim 21 wherein the demodulator comprises a QPSK demodulator.42. The optical communications system of claim 21 wherein thedemodulator comprises one from the list of: a Quadrature AmplitudeModulation demodulator; a Frequency Shift Keying demodulator; anAmplitude Shift Keying demodulator; a Phase Shift Keying demodulator; anOn Off Keying demodulator; and a BiPhase Shift Keying demodulator. 43.The optical communications system of claim 41 wherein the demodulatorcomprises: an input for the local oscillator; a first variable phasedelay coupled to the input for the local oscillator; an I-channel mixercoupled to the first variable phase delay and coupled to receive thesubcarrier channel, for demodulating the subcarrier channel to producean I-channel; a second variable phase delay coupled to the firstvariable phase delay and centered at 90 degrees phase; a Q-channel mixercoupled to the second variable phase delay and coupled to receive thesubcarrier channel, for demodulating the subcarrier channel to aQ-channel; a Costas phase discriminator coupled to the I-channel mixer,the Q-channel mixer, and the first variable phase delay, configured tovary the phase delay of the first variable phase delay to correct aphase imbalance between the I and Q channels and the subcarrier channel,and a quadrature imbalance compensated phase discriminator coupled tothe I-channel mixer, the Q-channel mixer, and the second variable phasedelay, configured to vary the phase delay of the second variable phasedelay to correct phase imbalance between the I channel and the Qchannel.
 44. A method for transmitting a data stream using an opticalcommunications system, the method comprising: generating a pilot tone ata pilot tone frequency; generating a reference clock signal at areference clock frequency; receiving a subcarrier channel, wherein thesubcarrier channel includes a coherently modulated data stream and islocated at a subcarrier frequency; generating a reference signal byusing the reference clock to modulate the pilot tone; combining thegenerated reference signal and the subcarrier channel into aninformation signal; and generating an optical signal from theinformation signal, the optical signal including the generated referencesignal and the subcarrier channel; wherein the step of combining thegenerated reference signal and the subcarrier channel comprises:combining additional subcarrier channels into the information signal,wherein each subcarrier channel includes a coherently modulated datastream and is located at a unique subcarrier frequency.
 45. The methodof claim 44 wherein in generating the optical signal, the opticalmodulator preserves the phase relationship between the pilot tone andsubcarrier.
 46. The method of claim 44 wherein the step of combining thegenerated reference signal and subcarrier channel comprises: combiningthe reference signal and subcarrier channel into the information signal;wherein within the information signal the frequency space occupied bythe reference signal does not overlap with the frequency space occupiedby the subcarrier channel.
 47. The method of claim 44 wherein adifference between the pilot tone frequency and the subcarrier frequencyis a fractional multiple of the reference clock frequency.
 48. Themethod of claim 47 wherein the fractional multiple is predetermined. 49.The method of claim 44 wherein the step of combining the generatedreference signal and subcarrier channel further includes: combining thereference signal and the subcarrier channels into the informationsignal.
 50. The method of claim 44 wherein a difference between thepilot tone frequency and each subcarrier frequency is a fractionalmultiple of the reference clock frequency.
 51. A method for recovering adata stream using an optical communications system, the methodcomprising: receiving an optical signal; converting the received opticalsignal to an electrical information signal using heterodyne detection,the electrical information signal comprising a pilot tone at a pilottone frequency, a reference clock signal at a reference clock frequency,and a subcarrier channel located at a subcarrier frequency; recoveringthe pilot tone, the reference clock signal, and the subcarrier channelfrom the information signal; generating a local oscillator from thepilot tone and the reference clock signal; and demodulating thesubcarrier channel using the local oscillator, wherein phase noisecontained in the local oscillator is correlated with phase noisecontained in the subcarrier channel.
 52. The method of claim 51 whereinthe step of demodulating the subcarrier channel comprises: coherentlydemodulating the subcarrier channel.
 53. The method of claim 51 whereinthe step of recovering the pilot tone, the reference clock signal, andthe subcarrier channel from the information signal comprises: matching agroup delay for the pilot tone and for the subcarrier channel.
 54. Themethod of claim 53 wherein the step of matching the group delay for thepilot tone and for the subcarrier channel comprises: matching a pathlength for the pilot tone and a path length for the subcarrier channel.55. The method of claim 51 wherein the step of recovering the pilottone, the reference clock signal, and the subcarrier channel from theinformation signal comprises: recovering the reference clock signalsubstantially free from phase noise.
 56. The method of claim 51 whereinthe step of recovering the reference clock signal comprises:incoherently demodulating the reference clock signal from the pilottone.
 57. The method of claim 51 wherein generating the local oscillatorcomprises: frequency multiplying the reference clock signal by afraction; and mixing the multiplied reference clock signal with thepilot tone to produce the local oscillator.
 58. The method of claim 51wherein generating the local oscillator comprises: generating the localoscillator at a frequency equal to the subcarrier frequency.
 59. Themethod of claim 51 wherein demodulating the subcarrier channelcomprises: substantially canceling the phase noise contained in thesubcarrier channel with the phase noise contained in the localoscillator.
 60. The method of claim 51 wherein: the electricalinformation signal further comprises additional subcarrier channelslocated at unique subcarrier frequencies; the step of recovering thepilot tone, the reference clock signal, and the subcarrier channel fromthe information signal further comprises recovering the additionalsubcarrier channels from the information signal; the step of generatinga local oscillator further includes generating a local oscillator foreach additional subcarrier channel; and the step of demodulating thesubcarrier channel further includes demodulating the additionalsubcarrier channels, wherein phase noise contained in each additionallocal oscillator is correlated with phase noise contained in thecorresponding additional subcarrier channel.
 61. The method of claim 60wherein generating each additional local oscillator comprises: frequencymultiplying the reference clock signal by a fraction; and mixing themultiplied reference clock signal with the pilot tone to produce eachadditional local oscillator.
 62. The method of claim 60 whereingenerating each additional local oscillator comprises: generating thelocal oscillator at a frequency equal to the subcarrier frequency of acorresponding additional subcarrier channel.